You don’t have to learn or do everything for your brand yourself. You can hire other people to help you create the designs you envision. However, if you want to improve your technical fashion design skills, there is one software I recommend you learn.
Adobe Illustrator is still the most useful program for designers to know. Even in the age of AI and 3D, fashion flat sketches done in Adobe Illustrator are still the industry standard for communicating design ideas. Plus, Illustrator can be used for so many other things for your brand – like creating branding graphics, social posts, garment labels, print artwork, and more.
Even if you can’t draw with pencil and paper or aren’t great with technology, Adobe Illustrator is definitely worth learning if you have any interest. Focus on learning just the tools you need for what you want to do for your brand.
(If you want to get started learning Adobe Illustrator just for the fashion industry, great places to start are Youtube tutorials from Sew Heidi, Zoe Hong, and 383 Design Studio.)